1. Field of the Invention
The present invention relates to a bias voltage generating circuit and an amplifier circuit. In particular, the invention relates to a technology for controlling bias circuits.
2. Description of the Related Art
Recently, cellular phones have incorporated a variety of additional functions such as an image shooting function, an image reproducing function, a moving image shooting function, and a moving image reproducing function. Then, reduced power consumption is increasingly required of built-in amplifier circuits. In particular, one-chip LSIs containing analog-to-digital converters (hereinafter, referred to as an “AD converter”) are becoming finer year by year, requiring a further reduction in power supply voltage. Multistage pipelined AD converters are known as AD conversion circuits of high conversion accuracy (for example, see Japanese Patent Laid-Open Publication No. Hei 9-69776, its entirety and FIG. 1). Moreover, AD converters contain operational amplifiers, and there have been known techniques for accelerating the rise of the output voltages of circuits that apply biases to the operational amplifiers (for example, see Japanese Utility Model Laid-Open Publication No. Hei 5-17712, its entirety and FIG. 1).
The amplifier circuits mentioned above usually keep applying the bias voltages irrespective of the types of their operations, thus consuming power more than necessary. Meanwhile, the bias voltages can be turned off for a relatively long time during a standby period in which no power is required, whereas capacitors or other components must be provided to reduce leakage during the off period. This has caused a problem of increased circuit area.